The present invention relates to a pneumatic tire having an improved non-directional tread pattern.
Recently, a non-directional tread pattern which is generally symmetrical about a point on the tire equator is widely used in passenger car tires.
In the non-directional tread patterns, axial grooves extending axially inwardly from the tread edges (E) are inclined to the same direction with a left side upward inclination as shown in FIG. 2 or a right side upward inclination as shown in FIGS. 4(a)-(b).
In general, the inclination angle (xcex2) of the axial grooves (a) and (b) to the tire circumferential direction is set in the range of more than 50 degrees as shown in FIGS. 4(a)-(b).
On the other hand, in the passenger car tires, a relatively low aspect ratio is widely employed nowadays. In other words, a relatively wide tread where drainage from the central part thereof is difficult is employed.
If the number and/or width of the axial grooves (a) and (b) which extend from the tread central part to the tread edges (E) are increased, the drainage can be improved, but the pitch noise is inevitably increased. Thus, such measures can not be taken. The pitch noise is a noise generated from the blocks (c) which periodically contact with the road surface during running.
The present inventors tried decreasing the inclination angle of the axial grooves to a value in the range of less than 45 degrees which is considerably smaller than the above-mentioned conventional range, and found that the drainage can be increased without increasing the axial grooves.
However, the blocks between the axial grooves are liable to decrease in the rigidity. Thus, the blocks are liable to slip against the road surface during running. Such slippage not only decreases the steering stability but also generates another type of noise (hereinafter slip noise) having a sharp peak (e) at about 1.6 KHz as shown in FIG. 3 in solid line.
Therefore, a primary object of the present invention is to provide a pneumatic tire having a non-directional tread pattern in which the inclination angle of axial grooves to the circumferential direction is decreased to a relatively small value to improve the wet performance.
Another object of the present invention is to improve the steering stability and slip noise by controlling the slippage of the blocks by specifically limiting the cord inclination of a radially outermost belt ply with relation to the inclination of the axial grooves.
As inclination angle of the axial grooves is considerably decreased as explained above, another problem arises. In such a non-directional tread pattern, the residual cornering force (CF) is liable to increase, and the problemxe2x80x94drifting to one side of a vehicle arises.
Therefore, still another object of the present invention is to control the residual cornering force to prevent the drifting to one side of a vehicle without sacrificing the wet and dry running performance.
Furthermore, it was found that the steering stability can be improved by increasing the circumferential pitch lengths of the axial grooves. However, if the pitch lengths are increased, as the number of the axial grooves decreases, wet performance decreases.
In general, on the other hand, the above-mentioned pitch noise can be reduced by employing variable pitches. However, if the pitch lengths are increased, the pitch noise reduction also becomes ineffectual.
Therefore, still more another object of the present invention is to improve the steering stability without sacrificing the wet performance and pitch noise by increasing the pitch lengths in the tread middle part but decreasing the pitch lengths in the tread shoulder part.
According to one aspect of the present invention, a pneumatic tire comprises a tread portion with tread edges, a pair of sidewall portions, a pair of bead portions, a carcass extending between the bead portions, and a belt disposed radially outside the carcass in the tread portion, the tread portion provided on each side of the tire equator with axial grooves extending axially inwardly from the tread edges, the axial grooves inclined to the same direction with respect to the tire axial direction to form a non-directional tread pattern, the inclination angle of the axial grooves with respect to the tire circumferential direction is decreased from the axially outer end to the axially inner end thereof, wherein the minimum inclination angle of each axial groove is less than 45 degrees, and a maximum inclination angle is 60 to 85 degrees.